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My local inspector is doing some code research for me concerning required duct sealing options but since code is the minimum standard, I would like to know the best method(s) to ensure an air tight duct system. For instance, in an extended/graduated plenub comprised of hard round pipe, how should the connections be sealed? What about branch offs of flex? What about insulating options for round pipe? Note: this is a crawlspace installation.

We use hard pipe and we tape each connection with foil tape and insulate everything but the return in a crawlspace, but we do tape all connections of the return and we also use duct sealer on the return. It works great with no duct leakage we have done a blower door test on a couple of jobs and have to shut it down pretty quick.

Anything you use needs to be UL-181 approved. A lot of people use mastic duct sealant, its a white water based compound you apply with a short bristle brush. There is a foil tape with a butyl rubber on one side. (foilgrip 1402)Carlisle makes both products.http://www.hardcast.com/PRODUCTS/pr_detail.asp?pid=3

Bldg. scientist recognize only duct sealing mastic. Tapes often don't stick to the oily metal or the fall off as the acrylic adhesive breaks down over time. Go to http://www.infiltec.com and read about duct sealing in the links section.
HTH

I appreciate the responses. Sounds like foil tape ( the mastic type ) is acceptable. My contractor actually brushes mastic over the foil tape, though it got overlooked on my job. However, when I inquired about what was proper, he insisted on going back and putting mastic over all the foil taped joints. He says thats how they normally do it and his mechanic just dropped the ball.